a) Field of the Invention
This invention relates to a nucleus material for pearls, which permits production of high-quality pearls in a good yield.
More specifically, the invention is concerned with an artificial nucleus for a pearl, said artificial nucleus having been produced from a batch of a specific composition composed of oxides and a fluoride, containing crystalline phases composed primarily of tetrasilicon fluormica and formed in a glass matrix phase, and having excellent forming ability of pearl layers, i.e., nacreous layers, a production process of the artificial nucleus, and a pearl produced using the artificial nucleus.
b) Description of the Related Art
Pearls are physiologically produced within bivalves led by Akoya pearl oysters (Pinctada fucara (Gould)). In the case of cultured pearls, in particular, they are each produced by inserting into a mother oyster a piece (a small segment of the mantle of a mother oyster) and as a nucleus, a material chosen by human and then culturing the mother oyster for a predetermined period to have a substance formed as nacreous layers on a surface of the nucleus as a result of metabolic secretion by the mother oyster.
To produce peals of good quality in a high yield, it is therefore extremely important to choose an appropriate nucleus material (natural or artificial nuclei) for insertion into mother oysters.
Production of cultured peals requires a nucleus material which can meet such conditions as will be mentioned below.
(1) Economy-related conditions
The nucleus material has uniform quality and can be supplied in various sizes, in large quantity and at low price.
(2) Physiology-related conditions
When inserted into the bodies of mother oysters, the nucleus material is not supposed to cause any substantial problem in the metabolic secretion and the like of the mother oysters. More specifically, it is preferred to satisfy the following conditions:
(a) Good bio-activating property. PA1 (b) Good surface smoothness. PA1 (c) No bio-harmfulness. PA1 (a) Good color shade and transparency: PA1 (b) Excellent structural properties PA1 (c) Ability to produce pearls with excellent borability PA1 (d) Ability to produce pearls with excellent chemical resistance PA1 (e) Ability to produce pearls with excellent durability of commercial value PA1 (f) Ability to produce pearls having a specific gravity equal to those produced using natural nuclei PA1 (i) anisotropic, crystallized glasses each obtained by melting compounds of mica component elements, such as aluminum, magnesium, alkali metals, fluorine and boron, together with silicon dioxide and heat-treating the melt to allow fine crystals of a synthetic mica to grow at random in a glass so that the crystallized glass is formed of a glass and a ceramic; PA1 (ii) crystallized glasses each obtained by dry pressing with a calcium compound such as calcium silicate or calcium phosphate, a magnesium compound and the like being contained in large amounts, followed by firing; and also discloses, as preferred examples: PA1 (iii) free-cutting, crystallized glasses having an SiO.sub.2 content not greater than 50 wt. % and having a Knoop hardness number not greater than 20. PA1 SiO.sub.2 . . . 40-65 wt. % PA1 MgO . . . 3-16 wt. % PA1 MgF.sub.2 . . . 6.5-17 wt. % PA1 K.sub.2 O . . . 7.5-18 wt. % PA1 ZrO.sub.2 . . . 0.01-15 wt. % PA1 CaO . . . 0.1-20 wt. % PA1 P.sub.2 O.sub.5 . . . 0.1-20 wt. % PA1 BaO and/or SrO . . . 0.1-13 wt. % PA1 SiO.sub.2 . . . 40-65 wt. % PA1 MgO . . . 3-16 wt. % PA1 MgF.sub.2 . . . 6.5-17 wt. % PA1 K.sub.2 O . . . 7.5-18 wt. % PA1 ZrO.sub.2 . . . 0.01-15 wt. % PA1 CaO . . . 0.1-20 wt. % PA1 P.sub.2 O.sub.5 . . . 0.1-20 wt. % PA1 BaO and/or SrO . . . 0.1-13 wt. %; PA1 SiO.sub.2 . . . 25-75 wt. % PA1 MgO . . . 0-20 wt. % PA1 F . . . 0-12 wt. % PA1 K.sub.2 O . . . 5-30 wt. % PA1 ZrO.sub.2 . . . 5-35 wt. % PA1 CaO . . . 0.1-40 wt. % PA1 P.sub.2 O.sub.5 . . . 0.1-40 wt. % PA1 BaO and/or SrO . . . 0.1-20 wt. %.
The condition (a) is needed for the formation of nacreous layers in a short time, that is, for high productivity. On the other hand, the conditions (b) and (c), when met, led to the prevention of any harmful impetus to mother oysters or any reduction in the metabolic secreting function of the mother oysters, in other words, to the avoidance of death or de-nucleation (i.e., loss of nuclei) of the mother oysters and hence to the production of high-quality pearls in a good yield.
(3) Physical-property-relate conditions
The nucleus material is required to be excellent in the productivity of nuclei (i.e, the workability upon being worked or machined into true spheres, being surface finished and the like) and to permit production of pearls having excellent workability and superb adaptability as merchandise. More specifically, it is preferred to meet the following conditions:
The nucleus material to be employed is required to have uniform whitish translucence without a color or striped pattern and to permit production of peals free of localized excess luster which will occurs when a nucleus material having an uneven color or a high light transmission is used. PA2 The nucleus material is required to be free of cleavage or cracks and, upon formation and working into nuclei, to permit ready formation into nuclei having high surface smoothness and true sphericity while assuring a good yield upon production of the nuclei and working of resulting pearls. PA2 Upon boring pearls in the production of a necklace or the like, the pearls are supposed to permit easy machining without cracking or flaking of their nacreous layers so that a high yield can be achieved. PA2 The resulting pearls should have good resistance to hydrogen peroxide, alcohols, ketons, hydrocarbon solvents and the like and, when subjected to destaining chemical treatment upon their working, should not develop any problem. PA2 Nacreous layers should not undergo discoloration or embrittlement due to surrounding conditions or aging and should remain free from flaking, so that the pearls have high stability as merchandise. PA2 In the distribution or transaction of pearls, the pearls are presently transacted in weight as a matter of fact. As a standard for their weight, the specific gravity of pearls obtained using natural nuclei (i.e., nuclei produced from shells of natural shell fish such as Tenshin freshwater mussel (Lamprotula spp.), namely, 2.85.+-.0.10 is adopted nowadays. It is therefore important to make artificial nuclei reflect this fact upon development of the artificial nuclei. PA2 (Note) The above proposal specifically discloses as the above-described crystallized glass "MACOAL" (trade mark; product of Corning, Inc., U.S.A.). PA2 (Note) From the above description, the exact composition of the crystallized glass is unknown. The present inventors however believe that the composition of the above-described crystallized glass and the structure of its crystalline phases are of the same kinds as those disclosed in U.S. Pat. Nos. 3,689,293 and 3,732,057 and Japanese Patent Application Laid-Open (Kokai) No. SHO 62-7649. A description will be made subsequently herein about the structure of the crystalline phases of the crystallized glass.
Needless to say, to furnish artificial nuclei for cultured pearls, it is therefore essential to develop a material which can meet the various conditions described above.
A description is now made of nucleus materials proposed to date for cultured pearls. At the dawn of pearl cultivation, use of a variety of materials was attempted, led by lead shotgun bullets, dried mullet roe, poppy seeds, hard paraffin, rosin and gutta-percha (natural rubber-like resin) and including silver, marble, coral, porcelain and the like. Since 1915 or so, natural nuclei made of shells have been considered to be most suited. As nuclei for cultured peals, natural nuclei obtained by grinding and polishing natural shells, which are in turn obtained from Tenshin freshwater mussel (Lamprotula spp.), into globules are still used widely these days.
It is however difficult to obtain nuclei of a uniform color shade and structure from natural shells. In addition, their size is limited by the thickness of the natural shells. It is therefore the current situation that nuclei of 10 mm or greater are practically not available. Moreover, Tenshin freshwater mussel (Lamprotula spp.) is on the decrease and is becoming smaller all over the world due to the overgathering to date, so that the nucleus material is increasingly becoming higher in price, smaller in diameter and lower in quality.
With the foregoing circumstances in view, attempts have been made, in the field of pearl culturing, to use as nuclei (artificial or synthetic nuclei) artificially-produced formed bodies in place of natural nuclei made from shells. As artificial nuclei of this sort, various artificial nuclei have been proposed as will be described below.
(1) Artificial nuclei making use of a synthetic resin
Artificial nuclei made of a synthetic resin have a uniform color shade and can be supplied in various sizes, in large quantity and at low price. They however involve may problems such as the toxicity to the bodies of mother oysters, the difficulty in boring upon working of pearls, the poor resistance to destaining chemical treatment, the susceptibility of nacreous layers to discoloration, embrittlement and/or flaking due to surrounding conditions or aging, and the difference in specific gravity. Accordingly, the National Federation of Pearl Cultivators Cooperated Association banned transaction of pearls making use of such nuclei in September, 1958.
(2) Artificial nuclei, namely, so-called kneaded nuclei obtained using a material formed of a synthetic resin and various fillers including shell powder mixed therein
For example, Japanese Patent Application Laid-Open (Kokai) No. SHO 48-52594 proposed a production process of artificial nuclei for cultured peals, which have a desired diameter of 10 mm or smaller and a smooth spherical surface. According to the process, 10 parts of a waterproof adhesive or a waterproof synthetic resin are added to 10-50 parts of powder of precipitated light calcium carbonate obtained by chemical treatment, and the resultant mass is granulated.
In the above-described proposal of synthetic resin nuclei, it is stated that such synthetic resin nuclei permit easier boring upon working pearls and use of a resin having low toxicity can improve the toxicity to mother oysters.
In pearls obtained using such a kneaded nucleus, their nacreous layers are prone to flaking due to aging and temperature fluctuations over years and upon boring, the nacreous layers tend to develop cracks. According to Japanese Patent Application Laid-Open No. (Kokai) No. SHO 63-219325, in particular, the above problems have been found to be attributed to the coefficient of linear expansion of the nucleus material. This patent publication indicates that such problems can be overcome by controlling the coefficient of linear expansion of a nucleus material within a range of 0.5-2.0.times.10.sup.-5 /.degree. C.
(3) Artificial nuclei proposed recently and making use of a ceramic material
Japanese Patent Application Laid-Open (Kokai) No. SHO 60-259135, for example, discloses a culturing method of colored pearls. This method uses, as artificial nuclei, formed bodies which have a globule size of 20 mm or smaller and have been obtained by mixing 20-100 wt. % of powder having a particle size of 0.1-105 .mu.m and selected from pearl powder, coral powder or shell powder of a crustacea, 0-80 wt. % of calcium carbonate having a particle size of 1-100 .mu.m and 0-20 wt. % of an inorganic pigment and then forming the resultant mixture at 400.degree.-1500.degree. C. and elevated pressure.
Further, Japanese Patent Publication (Kokoku) No. HEI 2-12537, in view of the limited availability of resources for natural nuclei and limitations in characteristics, discloses a nucleus material for pearls, which is composed of a free-cutting crystallized glass having a Knoop hardness number of 50-250.
The proposal of Japanese Patent Publication (Kokoku) No. HEI 2-12537 referred to above is to use a free-cutting crystallized glass as a nucleus material for pearls with a view toward ensuring stability in supply and uniformity in quality and achieving an improvement in chemical resistance and machinability.
Described more specifically, the above-described proposal discloses the following illustrative examples of the free-cutting crystallized glass: